A battery energy storage system (BESS) typically comprises a power converter, such as a STATic VAr COMpensator (STATCOM), and batteries connected to one or more direct current (DC) link capacitor(s) of the power converter. The batteries enable the power converter to consume and deliver reactive power as well as active power to an electric power grid at the point of connection.
The batteries often comprise electro-chemical batteries, which have a varying output voltage. For such batteries the output voltage may vary 30% or even more during a full charge-discharge cycle. If the batteries are connected directly in parallel with the DC link capacitor of the power converter, the DC link voltage of the power converter will be determined by the battery voltage and hence vary during discharge of the batteries. This requires a de-rating of the power converter that is proportional to the battery voltage variation.
In a BESS that has an active power rating (MW-rating) that is equal or close to its apparent power rating (MVA-rating) such de-rating could possibly be acceptable. However, in a BESS that has a MVA rating that is significantly higher than the MW rating, the cost for de-rating the power converter can become large compared to the active power that is added.
A known way of avoiding de-rating is to use a DC/DC boost converter, or a DC-DC buck converter, or a DC-DC buck-boost converter, which is arranged to keep the converter's DC-link voltage constant. However, such solution has a disadvantage in that with a constant load current from the load, the discharge current of the battery will increase during discharge and hence provide a larger voltage drop. The battery therefore has to be dimensioned to be able to handle this increased current, which increases the cost.